Huntington's Disease (HD) is one of eight progressive neurodegenerative disorders in which the underlying mutation is a CAG expansion encoding a polyglutamine tract. The strong dependence of toxicity on the CAG repeats number in HD and its late onset has raised the possibility that stopping expansion at the DNA level may be a promising therapeutic strategy. We have followed CAG expansion in germ cells and in somatic tissues of transgenic mice harboring a fragment of the human HD (hHD) gene. Surprisingly, we have shown that Msh2 plays a causative role; crossing hHD (-/+) with Msh 2(-/-) mice results in complete abrogation of expansion in the progeny. The finding that loss of Msh2 attenuates expansion in animals provides the first evidence that expansion can be stopped in vivo. These data have raised hope that a therapeutic intervention of a repair complex could be used to prevent, attenuate or at least delay onset of disease. Our data suggest a model in which Msh2 exacerbates the expansion mutation by binding and stabilizing CAG hairpins. It is the overall aim of this proposal to determine why hairpin interaction with Msh 2 causes rather then corrects expansion We propose tests of the hypothesis that interaction of a normal Msh 2 complex with a stably hydrogen bonded CAG hairpin impairs recognition aborting proper repair. We will genetically define if canonical MMR proteins are causing expansion by crossing hHD CAG mouse with appropriate knockout animals of Msh 2,3,6, and PMS 1 and 2. We will correlate the timing of expansion with the presence of implicated heterodimers during animal development. Using purified human enzymes, we will evaluate whether interaction of an MSh 2 complex with a hairpin impairs binding or enzymatic activity required for proper Msh 2 recognition (including binding, dimerization, nucleotide exchange and ATP hydrolysis). Successful completion of the aims will (1) identify the nature of the MMR complex that causes expansion, (3) define the step of MMR recognition that is impaired (binding, dimerization, enzymatic function) (2) establish if the MMR defect is an abrogation of a canonical or non-canonical mismatch pathway, (4) begin to identify the non-canonical pathway, if relevant, (4) shed light on the expansion mechanism, and (4) start, if possible, drug design.